1. Field of the Invention
The present invention relates to a multilayer wiring device and to the insulating film structure thereof.
2. Description of the Related Art
It has long been known that increased parasitic capacitance of an insulating film leads to decreased signal transmission speed, but when the wire spacing of semiconductor devices was over 1 μm, the effect of wiring delay on the device as a whole was small.
However, the effect on device speed is greater when the wire spacing is 1 μm or less, and in the future when circuits are formed with a wire spacing of 0.1 μm or less, the parasitic capacitance between wires will have a much greater effect on device speed.
Specifically, as integrated semiconductor devices become more integrated and element density increases, and particularly as demand increases for more multilayered semiconductor elements, the wire spacing is becoming narrower as the devices become more integrated, and wiring delay due to increased capacitance between wires is becoming more of a problem. Wiring delay (T) is affected by wiring resistance (R) and capacitance between wires (C), as shown by Formula (3) below.T∝CR  (3)The relationship between ε (permittivity) and C in Formula (1) is shown by Formula (4).C=ε0εrS/d  (4)(wherein S is the electrode area, ε0 is the permittivity in vacuum, εr is the permittivity of an insulating film and d is the wire spacing).
Consequently, an effective means of reducing the wiring delay is to reduce the permittivity of the insulating film.
At present, the principal kinds of insulating films used in the multilayer wiring structures of semiconductor devices and other multilayer wiring devices are low-permittivity coated insulating films and diffusion barrier insulating films and etching stopper layers formed by plasma CVD.
Inorganic films such as silicon dioxide (SiO2), silicon nitride (SiN) and phosphosilicate glass (PSG) or organic polymers such as polyimide and the like have conventionally been used as these insulating materials. However, the CVD-SiO2 films most commonly used in semiconductor devices have a relative permittivity as high as about 4. SiOF films, which are being explored as candidates for low-permittivity CVD films, have a relative permittivity of about 3.3 to 3.5, but are highly hygroscopic, and therefore permittivity tends to increase with time. Another recently-developed kind of low-permittivity film is produced by adding an organic resin or the like which is evaporated or decomposed by heat to a low-permittivity film-forming material, and then applying heat during film formation to produce a porous film, but because these are porous they are normally mechanically weak. Moreover, at present the pore size is large (10 nm or more), and if porosity is increased in order to reduce permittivity, permittivity is likely to rise and film strength to decline due to moisture absorption.
To solve this problem, methods are being studied for hardening and strengthening the insulating film by ultraviolet, plasma or electron beams after film formation, but in all these methods the insulating film is liable to increased permittivity and loss of thickness due to cleavage of Si—C bonds, which is observed as elimination of organic groups (principally CH3 groups), so an adequate solution has yet to be found. When the porosity of a porous insulating film is increased in an effort to reduce permittivity, the film becomes more hygroscopic, and the increase in permittivity of the insulating film due to Si—C bond cleavage tends to be more noticeable.
In an effort to control these damages and improve film strength without sacrificing permittivity (Japanese Patent Application No. 2004-356618, claims and Japanese Patent Application No. 2005-235850, claims), a method has been studied of forming a high-density insulating film on a porous insulating film and then exposing it to UV, plasma or electron beams from above, with some success, but more strength is still necessary for device applications.